This invention relates to an apparatus to compensate for a change in control point of a control system which results when the throttling range of the control system is changed.
In typical condition control systems, the output from a condition sensor is compared to a reference level established by a control point adjustment and an error signal is generated dependent upon the difference between the sensor output and the control point or setpoint adjustment output. This error signal is then used to control a load for, in turn, controlling the condition. The throttling range, sometimes referred to as the proportional band, of the system determines the impact that the error signal will have on the load. For example, in a control system where the load is a motor which may drive a valve, the throttling range of the system may be adjusted to insure that the motor will operate to its mid position as a result of a predetermined error signal. However, the throttling range can be changed to drive the motor to one of its extreme positions for the same predetermined error signal. Thus, the throttling range can be set for best system performance.
Although it is apparent that the incorporation of a throttling range adjustment in a control system has certain advantages, it does have at least one disadvantage in the typical control system. Specifically, a change in throttling range typically results in a change in the control point of the system. In such a system, when the throttling range is changed, the operator must also make a change in the control point of the system to compensate for the change in throttling range. However, the operator may not know how much to change the control point for a change in throttling range.
One prior art solution to this problem is to provide, in addition to the throttling range adjustment and the control point adjustment, a third element connected in circuit with the control point adjustment and the throttling range adjustment. The third element is operated in conjunction with the throttling range adjustment to compensate the system for any changes in throttling range. This third element and the throttling range adjustment are usually ganged potentiometers. In electrical control systems, for example, a bridge is arranged having the sensor, a setpoint potentiometer and a compensating potentiometer (the third element). The control system further includes a throttling range potentiometer for adjusting the throttling range of the system. The throttling range potentiometer and the compensating potentiometer are operatively connected together such that any change in the throttling range potentiometer also results in a change in the compensating potentiometer to maintain the control point constant when changes in throttling range are made.